Testing Soils for Tree Planting

When you decide to plant a tree there are many different things you must take into account before you purchase the tree.  You will need to answer a few key questions before you can figure out what type of tree you should plant in that area.

·         Will this tree be planted near a structure?
·         Will the tree be planted underneath utility lines?
·         What type of soils do you have at your site?
·         Do you have enough rain or supplemental water to grow a tree successfully?
·         What is the primary purpose of the tree? (shade, aesthetics, food production etc.)

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I am going to focus in this case on just one of those questions on this list and that is what type of soils you have at your site.  The soil that the tree will be planted in is one of the most overlooked factors when it comes to planting a tree.  The soil can play a critical factor in the success of a tree planting especially in in urban locations because of the altered state that urban soils typically exist in.  Urban soils are typically heavily modified by human activities and they can be highly compacted, have a high or low pH, nutrient poor or contaminated by many of the residues that come from human activities.
With all this in mind it is important to take a good look at the soil in your planting site to help you figure out what trees can grow there.  For example if the soil where you want to plant the tree has a high pH it is a alkaline soil and trees that prefer more acidic soil like maples will not grow well there.  Soil pH is just one of the factor to look at you should also consider, compaction, texture, water holding capacity and nutrient content name a few.



The three I am going to focus on here are,
·         Compaction
·         Texture
·         Water Holding Capacity



Compaction

Compaction can be tested in a number of both very accurate scientific methods and a few very simple methods that are more suitable for the average tree planter.  I am not going to get into the more scientific methods because they require specialized equipment and give you more information than you should really need.  What you need to know for the most part is whether or not the soil is compacted and that can be figured out fairly easily with shovel, a pin flag and your basic senses.

The first thing to do is look closely at the site,

·         Is there thin or sparse vegetation on your site when surrounding areas have thick good   vegetation?
·         Does water pool or puddle on the surface of the soil?

After looking at the site and observing what you see there the next step is probe around with a shovel.  I typically used a spade shovel and test the compaction simple by stepping/jumping on the back and seeing how far I go into the soil.  If you only go in a few inches and then stop suddenly you either hit a rock or you may have compacted soil.  Do this as many times as you need to in the area to get a feel for the site.  Compaction can be very localized and be in very small spots like a wheel ruts from a truck or it can cover a whole site like in a new housing development.
Another method to test for compaction is with the pin flag rod.  The easiest way to test with this is to dig a hole where you want to plant the tree and then use the rod to probe all sides of the hole.  In a minimally compacted soil the rod should go into the surrounding soil fairly easy.  Again you could run into rocks or other micro compaction spots so make sure to test all around the hole.



Soil Texture

The texture of a soil can be defined as the percent components of sand, silt and clay that are present in the soil.  The different amounts of these soil particles will determine the physical and chemical properties of the soil like,

·         Nutrient content
·         pH
·         Water Holding Capacity

So the texture of the soil which can change quickly from spot to spot is a critical part of knowing what trees can be planted there.  There are two easy ways to determine the texture of a specific soil.  The first one is the hand test that uses a soil texture triangle and a flowchart to guide you through some easy tests that will help you classify the soil.

The second test is even easier though it will take anywhere from a day to a couple of weeks to fully do.  Using a large jar take a good scoop or handful of soil and put it in the jar.  Fill the jar with water and fully mix up and stir the soil in until the soil is completely dissolved and suspended in the water.  Then put the jar somewhere level and let it sit at least 24 hours.  After the soil has settled out of the water you should be able to see distinct layers in the jar.  These layers are made up of sand, silts and clay that the soil is made up from.  You can simply measure the total thickness of the layer and then calculate the percentages by measuring the individual layers.

​Using a Soil Texture Triangle

Jar Test for Soil Composition



Water Holding Capacity

The water holding capacity of a soil will be a great determiner in what species will do well in a specific site.  Sandier soils will drain of water faster and will not hold as much in reserve from rainfall.  Heavy clay soils will hold a lot more water but they can be easier to compact and become waterlogged which can drown trees.  An easy way to test the soil for its water holding capacity it to dig a hole and fill it with water.  Then track how long it takes for that water to drain out of the hole and into the soil.  If the soil is dry from lack of rain you will need to fill it once wait for it to completely drain and then fill it again before you can measure how fast the drainage is.  There are tables you can use to calculate how fast the drainage is but you need dig a specific size of hole to do that.  Typically if the water drains within a couple hours on the second wetting you should be good to go for most types of trees.      

For more information like this check out other articles on the Green Living Library.

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